Unlicensed bands, such as the 2.4 GHz and the 5 GHz Industrial-Scientific-Medical (ISM) bands, have become crowded in recent years due to the increasing popularity of mobile communications and wireless technologies such as Wi-Fi, Bluetooth and mesh networks. In contrast, a large portion of the licensed bands remain under-utilized or even unused over time. For example, the average utilization of the licensed spectrum allotted for television (TV) broadcast was as low as 14% in 2004. Based on these observations, the Federal Communications Commission (FCC) recently agreed to reconsider the possible legal operation of unlicensed devices in “white spaces,” i.e., portions of the licensed TV bands that are not in active use by incumbent users, such as the TV broadcasters. This sub-GHz spectrum has several properties that make it desirable for data communication. In particular, its radio frequency (RF) communications can occur over longer distances and RF waves have better penetration properties in the lower bands compared to the higher frequency ISM bands.
The legitimate use of “commercial” media, such as TV broadcast bands, by unlicensed systems poses two major challenges. First, the unlicensed systems must not interfere with ongoing TV reception. Therefore, such a system should have a robust scheme for determining the white spaces, and second, these systems should have a spectrum-aware MAC protocol that efficiently utilizes white spaces of possibly varying bandwidths in available parts of the spectrum.